1. Field of the Invention
The invention relates generally to exhaust gas recirculation systems for emission control of internal combustion engines and is more particularly related to a closed loop EGR system programmable with engine operating parameters that is insensitive to environmental variations and aging.
2. Prior Art
It is conventional in the art to provide an exhaust gas recirculation (EGR) system for an internal combustion engine to reduce the emissions of NO.sub.x components. Known systems generally comprise a recirculation conduit connecting a source of exhaust gas to the induction source of the engine and a controllable valve device for regulating the amount of exhaust gas allowed to pass through the conduit.
The production of the NO.sub.x components in the emissions of an internal combustion engine is caused mainly by elevated combustion temperatures and pressures. EGR systems essentially reduce both of these to levels less conducive to the formation of the NO.sub.x components in two ways. Primarily exhaust gas contains numerous inert elements such as N.sub.2, H.sub.2 O, and CO.sub.2 which because of their molar specific heats will absorb substantial thermal energy from the operating cycle. The dilution of inducted air and fuel with a portion of exhaust gas will additionally cause proportionately less O.sub.2 in the resulting mixture and secondary cooling will occur from uncombusted fuel. With lower combustion temperature and smaller amounts of fuel being combusted, the combustion pressures will, of course, decrease also.
However, while the formation of the NO.sub.x components decreases with the recirculation of larger amounts of exhaust gas, the performance of the engine as measured by such parameters as power and driveability decreases also. Generally, for different operating conditions of the engine varying amounts of EGR will be optimum but it is understood that the maximum amount of dilution is in the range of 20 percent of the mixture constituents before driveability becomes unacceptable. Therefore, the problem exists of how to position the EGR valve to provide the maximum suppression of MO.sub.x constituents without affecting the performance of the engine over a wide range of operating conditions.
Most of the prior art attempts to solve this basic problem by mechanically positioning the valve in relation to a single operating parameter indicative of the amount of inducted mixture, one of the most advantageous of these parameters being manifold absolute pressure. Others include throttle position, RPM, air/fuel ratio, etc. By sensing an operating parameter and using it as a position indicator such a system contemplates mixing a constant percentage of exhaust gas with the inducted mixture at the various engine operating conditions. Generally, the constant is picked around the maximum recirculation amount of 20 percent and is not the optimum for all operating conditions.
Further, rather than the desired constant amount of EGR designed for, such a positioning scheme may actually give fairly unpredictable results because the amount of exhaust gas recirculation flow depends not only on the position of the valve but also significantly on the pressure in the exhaust manifold source. The range of speed and load conditions required of the modern engine will alter this pressure and the actual amount of EGR for any given position of the valve. Changes in altitude, barometric pressures, and the like can further cause discrepancies.
Other items which affect EGR differential valve pressure are tolerance, wear, and other aging conditions of any mechanical system. As one example, exhaust gas deposits or accumulations in the tail pipe can increase back pressures and accumulations on the valve itself will change the amount of EGR flow for the identical positionings of the valve at different times. Many systems provide compensation for some of these changing conditions by altering the position of the valve in combination with the main sensed parameter.
closed loop control systems for controlling various parameters of an internal combustion engine are known in the art, as are the above-identified EGR control systems, even though the prior art does not include many EGR control systems employing a closed loop mode for operation. U.S. Pat. No. 3,872,846 issued to L. B. Taplin et al. on Mar. 25, 1975 for an Exhaust Gas Recirculation (EGR) Internal Combustion Engine Roughness Control System is one example and is assigned to the assignee of the present invention. The disclosure of Taplin is hereby expressly incorporated herein by reference. This patent teaches a closed loop internal combustion engine control system provided for controlling the exhaust gas recirculation flow so as to regulate engine roughness at a predetermined level. The closed loop control system of this patent receives input signals indicative of the engine roughness and generates therefrom an EGR value command signal for varying the position of an EGR valve so as to effect the maximum possible EGR flow compatible with a predetermined maximum level of permissible engine roughness.
Programmable EGR systems as noted in the above cross referenced applications are advantageously used for providing flexibility in positioning an EGR valve and regulating recirculation gas flow in a closed loop manner. However, an improved EGR system could be developed if the actual EGR flow could be measured and combined with such a programmable closed loop control.